Method for use of inorganic azides as photosensitive materials at low temperatures

ABSTRACT

A new method is provided for the production of permanent photographic images at low temperatures by the use of heavy metal azides such as thallium, lead and silver azide as the photosensitive material in the photographic process.

United States Patent Wiegand [75] Inventor: DonaldA. Wiegand, Landing, NJ.

[73] Assignee: The United States of America as represented by the Secretary of the Army, Washington, D.C.

[22] Filed: Nov. 19, 1971 [21] Appl. No.: 200,616

[52] US. Cl 96/27 R, 96/49, 96/50 R, 96/88, 96/91 N, 96/94 BF [51] Int. Cl. G03c 5/00, G030 5/16, G030 5/18 [58] Field of Search 96/27 R, 48 R, 50 R, 96/88, 91 N, 49, 94 R, 94 BF [56] References Cited UNITED STATES PATENTS 2,414,839 l/1947 Schoen 96/88 X 3,316,088 4/1967 Schaffert 96/48 X 3,298,833 1/1967 Gaynor 96/27 R 1,880,503 10/1932 Sheppard 96/88 2,019,737 11/1935 Sheppard et al... 96/88 2,095,839 10/1937 Sheppard et al 96/91 N X Dec. 11, 1973 2,703,283 3 1955 Eggert 96/88 x 3,377,169 4/1968 Blake 96/48 R X OTHER PUBLICATIONS Kosar, J., Light-Sensitive Systems," Wiley & Sons, 1965, p. 15-21. Glafkides, P., Photographic Chemistry," Fountain Press, 1958, p. 32-33. Mees, C. E. et al., The Theory of the Photographic Process," 3rd Ed., McMillan Co., 1966, p. 144447. Fair et al., .1. Phys. Chem. Solids, Pergamon Press, Vol. 30, 1969, p. 2,559-2,570. Brown, F. C. et al., J. Phys. Chem. Solids, Pergamon Press, Vol. 22, 12/1961, p. 101-107.

Primary Examiner-Charles L. Bowers, Jr. Attorney-Harry M. Saragovitz et al.

[57] ABSTRACT A new method is provided for the production of permanent photographic images at low temperatures by the use of heavy metal azides such as thallium, lead and silver azide as the photosensitive material in the photographic process.

4 Claims, No Drawings METHOD FOR USE OF INORGANIC AZIDES AS PHOTOSENSITIVE MATERIALS AT LOW TEMPERATURES The invention described herein may be manufactured, used and licensed by or for the Government for governmental purposes without the payment to me of any royalty thereon.

BACKGROUND OF THE INVENTION For certain applications in the field of photography such as photographs in outer space, in certain scientific investigations and in military work it is desirable to have a reliable method of producing images at extremely low temperatures.

Most photographic emulsions have, as the photosensitive material, one of the silver halides, e.g., Ag Br or Ag Cl. Although there are many other photosensitive materials the present comparison is limited to the silver halides. On exposure to radiation the silver halides decompose with the formation of Ag metal and, at least under some conditions, the evolution of halogen gas. The photographic image is formed by the Ag metal. The silver halides are used in emulsions because of their sensitivity to light, because of the stability of the products, and because of the ability to be developed. The latter process involves the conversion of a grain of silver halide, which has absorbed a minimum critical number of photons, completely into silver metal by chemical means. It is well known that the sensitivity of these emulsions and indeed the sensitivity of the pure silver halides decrease markedly with decreasing temperatures below room temperature. For emulsions containing sensitizing dyes, the loss of sensitivity with decreasing temperature when exposed to light the absorption band of the dye is usually greater than when exposed to light in the absorption band of the silver halide. Additionally, spectrally sensitized emulsions exposed at temperatures below about 200K and developed at room temperature are nearly always sensitive in the specitrally sensitized region than when they are exposed and developed at room temperature. Due to this desensitization, available silver halide photographic emulsions, are unsatisfactory at extremely low temperature. This insensitivity results in either no image production at all or in a very slow image production.

It is, therefore, an object of the invention to provide a photographic system for use at low temperatures.

A further object is to provide a photosensitive component for use in low temperature photography.

Another object is to provide a faster, reliable, low temperature photographic process for the production of permanent images.

Other objects and many of the attendant advantages of this invention will be readily appreciated as the same become better understood by reference to the following description, wherein it is shown that the abovementioned objects are attained and the prior art deficiencies are overcome by the use of heavy metal azides as the photosensitivie component of a photographic process for low temperatures. Although the optical and electrical properties of silver azide have been thoroughly explored in an articles by McLaren, A. C. and Rogers, G. T. in Proceedings of the Royal Society A 240, page 484 (1957) low temperature image production has not been recognized.

DESCRIPTION OF A PREFERRED EMBODIMENT l have found that, on exposure to irradiation Tl, Pb and Ag azides decompose with the formation of colloidal metal and at temperatures above about K with the evolution of nitrogen gas. Thus, radiation produces essentially the same effects in these azides as in the silver halides. However, these azides are much more sensitive to irradiation at low temperatures, e.g. 12K and 78K, than the silver halides.

Films of the azides are prepared using the method of Fair and Forsyth J. Phys. Chem. Solids 30 2559-2570 Pergamon Press 1969 by reacting vacuum deposited layers of metal with gaseous hydrazoic acid for several hours.

Films of metallic lead are vacuum deposited on quartz or sapphire substrates. The: substrates are in the form of discs 0.5 in. dia. and 0.02 inch thick. The films are then placed in a teflon holder inside a light tight desiccator which is attached to the output of a hydrazoic acid generator. This desiccator is flushed continuously with hydrazoic acid in a nitrogen carrier gas.

After several hours of exposure to hydrazoic acid, the top surface of the metallic lead film becomes white. The lead surface next to the substrate appears to be grey when observed through a micrscope. Upon further exposure, the grey areas disappear and the lead films become either completely white or transparent depending upon the thickness of the initial lead film. Lead azide films up to 3,500 Angstroms thick may be produced by modifications of this process. Continued exposure to HN increases the optical quality of the film until after 4 hours of exposure the film is transparent to visible light. The most dramatic change in the visible part of the spectrum occurs shortly after the sample is exposed to hydrazoic acid.

The method of this invention which may be used to produce permanent photographic images at low temperatures is set forth in the following example. It is, of course, understood that this example is meant to be illustrative and not restrictive of my invention.

EXAMPLE Films prepared in the above manner are placed in a standard vacuum cryostat. Radiation opaque objects are placed over part of the film. The temperature is then lowered to a predetermined level. After the sys tem equilibrates the film is irradiated. This radiation is accomplished with a standard radiation source of variable wavelength, placed on the opposite side of the radiation opaque object. After a predetermined time of irradiation the film is observed both at low temperature and room temperature. The wavelengths used for different exposures are selected from the range of about 3,500 to 5,800 Angstrom units. The exposure time is varied between about 5 seconds and 600 seconds. The temperature is varied from about l2K to room temperature. The materials used for different exposures are lead azide, thallium azide and silver azide.

After irradiation of films at 78K in the above manner the colorations of T1, Pb and Ag azides due to colloidal metal formation were compared directly with the coloration of AgCl exposed at 78K. In all three cases the azide coloredmuch more rapidly than AgCl. In fact, the experiments failed to reveal any appreciable coloration of AgCl at this temperature. In addition, the coloration of TIN and AgN remained, on warming to room temperature, and the coloration of TIN; is stable to temperatures appreciably above room temperature.

TABLE 1 Comparison of observed and calculated values for change in absorption with change in wavelength of lead azide.

Observed "Calculated Wavelength in Optical Density Absorption in Angstrom Units Arbitrary Units 4500 0.17 1.46 5000 0.09 l.l7 5500 0.07 0.87 6000 0.06 0.76 6500 0.05 0.63 7000 0.04 0.66

calculated values are for lead spheres of 100 Angstrom diameter in a dielectric medium of N=2; "observed values are for lead azide irradiated at about 300K While the absolute value of the numbers is substantially different the general shape of the curve defined by these values is found to be quite similar.

Second, since nitrogen gas is given off during irradiation the sample must contain express metal which is most likely to exist in the colloidal state.

Third, that results of diffuse reflectance measurements which are insensitive to light scattering by imperfections and gas production indicate that the optical density as seen in transmission is due to absorption and not scattering. Absorption in turn is indicative of colloidal metal production.

Thus, it can be seen that the discovery has been made that TlN AgN and, under some conditions Pb(N satisfy two of the conditions necessary for use as photosensitive materials at low temperatures, i.e., sensitivity to irradiation (colloidal metal production) at low temperatures and stability of the products (colloidal metal) to elevated temperatures. The experimental results suggests that of the three azides, "UN, is the most suitable because it is apparently the most sensitive to irradiation and the least sensitive to explosive decomposition.

Thus it can be seen that the use of our invention, heavy metal azides as the photosensitive component of a low temperature photographic system, forms the basis for an advance in the art.

Since the colloidal metal is produced in the heavy metal azides by exposure to radiation it is reasonable to expect that inclusion of these azides into emulsions selected for their operability at low temperatures will result in the production of a photographic system. Additionally, modification of conventional methods of developing colloidal metal producing films may be found effective in developing images produced by the use of our invention.

I wish it to be understood that I do not desire to be limited to the exact details described, for obvious modification will occur to a person skilleclin the art.

1 claim:

1. A method for producing a permanent visible image comprising exposing a supported photosensitive layer containing a heavy metal azide selected from the group consisting of lead azide, thallium azide and silver azide to image-forming radiation of wavelength between about 3,500 and 5,800 Angstroms at a temperature below about 200K while partially masking said layer with a radiation shielding object, whereby the metal azide exposed to such radiation is converted at least in part to colloidal metal forming the permanent visible image.

2. The method of claim 1, wherein the temperature is below about K.

3. The method of claim 1, wherein the metal azide is thallium azide.

4. The method of claim 2, wherein the metal azide is thallium azide. 

2. The method of claim 1, wherein the temperature is below about 80*K.
 3. The method of claim 1, wherein the metal azide is thallium azide.
 4. The method of claim 2, wherein the metal azide is thallium azide. 